To design an asphalt paving mix, the proper amount of asphalt binder must be added to a given amount of aggregate material to maintain the right matrix of aggregate and binder in order to produce a paving material which will yield a strong and durable road. If there is too much binder in the mix, the road will be soft and rutting will occur. If there is not enough binder in the mix, the road will be brittle and will crumble or break apart.
Aggregates used in the preparation of asphalt for road construction are tested to determine the amount of asphalt binder that will be absorbed internally into the aggregate when a batch is prepared. When binder is absorbed internally into the porous aggregate, that absorbed binder does not contribute to the effective volume of the asphalt mix. In order to account for this, additional binder must be added, which essentially disappears in the mix. The measurement of the binder absorbed by the aggregate which does not contribute to the volume of the asphalt mix is the percent absorption, by weight, of water absorbed into the aggregate to the weight of the aggregate itself (“PA”).
The procedure for testing aggregate for PA is as follows. A sample of the dry aggregate is prepared to a condition where the internal voids are saturated with water, and the surface of the aggregate is dry. This condition is known as the saturated surface dry (“SSD”) state. The SSD sample is then weighed. The sample is then dried completely in an oven, and weighed again (dry). The difference between the SSD and dry weights, divided by the dry weight, and multiplied by 100, yields the PA.
The current method for determining whether aggregate is at SSD is what is known as the “slump” test. In this test, a sample of aggregate is prepared with excess water so that it is wetter than the SSD state. The aggregate is placed into a metal cone, the metal cone is placed atop a non-absorbent surface of a table or bench and the aggregate is tamped down into the cone, through an opening in the tip of the cone, with a metal tamper. With aggregate pieces having water on the surface, i.e. with the aggregate sample being wetter than the SSD state, the cone of aggregate will remain standing when the metal cone is removed. The water between the particles of aggregate holds the aggregate together, due to surface tension. The SSD point is reached when there is a “slight slump” of the aggregate when the metal cone is removed. Once the aggregate sample has been initially prepared to wetter than the SSD state the aggregate is progressively agitated and subjected to warm air flowing over it, repacked into the metal cone and the metal cone removed, until this slight slump occurs. A 500 gram sample is then taken from the SSD aggregate and weighed. The 500 gram sample is then completely dried in an oven and is weighed again. The PA is then computed from the two weights.
There are a number of problems with the slump test. First, the test is subjective. The definition of a “slight slump” will vary from technician testing the aggregate to the next. In addition, while the slump test works fairly well with natural sand, for which the test was originally developed, the test does not work as well for jagged material such as crushed granite and limestone. The crushed materials have a higher angularity (jaggedness) and a higher content of fine material, which packs better in the cone, holding the packed material together better. This requires the material to dry more before exhibiting a “slight slump”, making for an artificially dryer SSD point. On the other hand, a method which could actually measure the presence or absence of water on the surface of the aggregate would give a much more accurate measurement of whether the aggregate was in the SSD state or not and hence produce a much more accurate PA measurement.
Second, when the sample is at a temperature above room ambient, it will continue to lose water weight by evaporation as long as the sample remains on the table or bench. This produces an artificially low PA. Also, the time between reaching SSD and weighing the sample will not be consistent from batch to batch and technician to technician. If the sample could maintain its SSD condition/moisture content from the time that that condition is reached until the sample is weighed then the measurement would be more accurate and repeatable from batch to batch and technician to technician.
Third, as the sample is agitated and dried, the sample will begin to generate dust, which leaves the sample, and thus alters the aggregate constitution. Dust can also adversely effect mechanical parts such as bearings, motors, couplings etc. of the equipment used in the SSD/PA testing, thus contributing to premature failure of same. The dust is also a nuisance to the technicians operating the equipment. It would be desirable to somehow contain the dust generated by the sample during the SSD/PA determination.
Knowing the liquid absorption of a material is valuable for a variety of reasons. First, the liquid absorption relates to the optimum amount of time the material should be processed in the preparation of asphalt mixes and concrete mixes. Second, from the liquid absorption one can calculate the film coefficient, which relates to the Vssd, one of the parameters disclosed in the assignee's own U.S. Pat. No. 6,486,475, hereby incorporated by reference herein, which determines the SSD of the material.
Bulk specific gravity of an aggregate is defined as the weight of dry aggregate to the weight of weight having a volume equal to that of the aggregate including both its permeable and impermeable voids. Apparent specific gravity is defined as the ratio of dry aggregate to the weight of water having a volume equal to the solid volume of the aggregate excluding its permeable voids. One current method of determining the apparent specific gravity of a material sample involves soaking the material with water while manually hand agitating the material to remove air from the sample allowing water to displace the trapped air. Another current method of determining apparent specific gravity combines the step of pulling a partial vacuum on the vessel containing the specimen under test with manual hand agitation. Yet another current method has the technician pulling a vacuum on a pouch containing the sample to determine the apparent specific gravity, then puncturing the pouch under water to allow water into the sample to determine the liquid absorption. These methods are time consuming and prone to variation from one technician to the next.